This invention relates to curing agents for epoxide resins, a method for preparing such curing agents and a method for incorporating the curing agents into and subsequently curing epoxide resins. More specifically, this invention relates to nonadulterated, stable, liquid curing agents, their preparation and use.
In the past, aromatic polyamines have been used as curing or hardening agents for epoxide resins and compositions containing said resins. Since, however, such compounds are normally solid at room temperature, there are several inherent disadvantages attendant to their use as curing agents. For example, it has been the practice, when using an aromatic polyamine as a curing agent, to heat the amine above its melting point and then incorporate it into the resin system. By this method, the resin must also be heated to approximately the same temperature as the curing agent in order to preclude crystallization of the curing agent upon its addition to a cold resin system. Thus, by heating both the curing agent and the resin to a temperature above the melting point of the curing agent a mixture of the curing agent and the resin could be effected. Other methods which have been used in the past to incorporate polyamine curing agents into epoxide resin systems include: (1) dispersing the curing agent in the resin by grinding on a suitable mill, a method which is both costly and time consuming, (2) dissolving the curing agent in a solvent prior to incorporation into the resin, a procedure which necessitates later solvent removal steps or (3) preheating and liquifying the curing agent, a procedure which leads to possible uncontrolled exothermic reactions and extremely short pot life. Moreover, the curing agent when heated has a tendency to volatilize, thus presenting a problem in regard to the maintenance of proper proportions of curing agent and resin and also creating a health hazard.
Many attempts have been made to prepare stable, liquid curing agents of solid aromatic polyamines so that they might be easily incorporated into epoxide resin systems, however such attempts have been generally unsuccessful.
It has recently been proposed that a mixture of an aromatic polyamine, such as m-phenylene diamine, with p,p'-methyldianiline when heated forms a liquid curing agent composition which may be incorporated into an epoxide resin at room temperature and gives a moderately long pot life. Such mixtures, however, are not stable and have a rather high tendency to crystallize.